In a conventional power MOSFET, the source is often shorted to the body to prevent the parasitic bipolar transistor, consisting of the source, body and drain diffusions, from turning on as the MOSFET is operated. Bipolar turn-on is undesirable since it may result in a loss of voltage blocking capability (i.e., snapback), negative resistance which can cause oscillations, current crowding, hot spots and device destruction.
The source-body short is normally created by depositing a metal layer over both the source and body diffusions. As a result, an "antiparallel diode", represented by the junction of the body and drain, is formed in parallel with the channel of the MOSFET. This prevents the MOSFET from blocking current in the direction from the source to the drain, since such current would flow through the forward-biased antiparallel diode. Thus, to provide a bidirectional current blocking capability, two back-to-back MOSFETs are normally used. This back-to-back series arrangement doubles both the on-resistance and chip area of the device, resulting in a 4X penalty.
In trench-type MOSFETs, the source, body and drain regions are formed along the side of a trench which extends into the semiconductor material and which contains the gate. The current flows vertically through a channel adjacent to a wall of the trench. This offers the potential of a greater cell density and a lower resistance when the device is turned on. In trench MOSFETs, the source-body short is typically created by allowing the body diffusion to extend to the surface adjacent the source and depositing a metal layer over both diffusions.
Trench MOSFETs have suffered from the problem of excessive electric fields at the corners of the trenches, which can cause avalanche breakdown and thereby damage or rupture the gate oxide layer at these locations. U.S. Pat. No. 5,072,266 teaches the use of a deep body diffusion at the center of the cell to reduce the field strength at the gate oxide layer.
Both the need to create a source-body short at the surface of the device and the creation of a deep body diffusion tend to limit the cell density and consequently increase the on-resistance of the MOSFET.
Accordingly, there is a need for a bidirectional current blocking power MOSFET in which the gate oxide layer is protected against damage from electric field stress in a manner which does not limit the cell density of the device.